OpenAI has replaced GPT-5.3 Instant with a new foundation model, GPT-5.5 Instant, as the default engine powering ChatGPT. The company said the updated model significantly reduces hallucinations in high-stakes domains including law, medicine, and finance, while preserving the low-latency performance of its predecessor.
Benchmark results show meaningful gains, with the new model scoring 81.2 on the AIME 2025 math test against 65.4 for the older version, and outperforming it on multimodal reasoning. A key new capability allows the model to reference past conversations, files, and Gmail to deliver more personalised responses, initially available to Plus and Pro users.
For developers, GPT-5.5 Instant is accessible via API as “chat-latest,” with GPT-5.3 remaining available to paid users for three months.